4 ms·

sum(A) + sum(B) = sum(A ∪ B) sum(A) + sum({}) = sum(A ∪ {}) sum(A) + sum({}) = sum(A) sum({}) = 0

• dbaupp 13y ago
  (This assumes either A and B are multisets, or they are disjoint.)
  • kragen 13y ago
    Indeed, but in the context of minor changes in how things SQL should behave, we can assume that everything is a multiset.